Depot-specific expression of retinol-binding protein 4 in human adipose tissue and their relationship with obesity and insulin resistance

Abstract

To explore the depot-specific mRNA and protein expression of retinol-binding protein 4 (RBP4) in human subcutaneous and omental adipose tissue and investigate their relationship with the serum RBP4 levels, obesity and insulin resistance. A total of 41 subjects with normal glucose regulation were recruited. Among them, there were 29 cases with normal body mass index (BMI) and 12 cases with BMI ≥ 24 kg/m(2). All were prepared to undergone a selective abdominal surgery. The levels of serum RBP4 and fasting insulin (FINS) were measured by ELISA (enzyme-linked immunosorbent assay) and chemiluminescence ELISA kit respectively. Fasting plasma glucose (FPG) was tested by glucose oxidase and the home model insulin resistance index (HOMA-IR) calculated. Real-time PCR (RT-PCR) and Western blot were employed to assess the relative mRNA and protein expression of RBP4 in subcutaneous and omental adipose tissues. The mRNA and protein expression of RBP4 from different fat depots were compared and their correlations with BMI, the serum RBP4 concentrations and HOMA-IR analyzed. (1) The serum concentrations of RBP4, FINS and HOMA-IR were significantly higher in overweight and obesity group than those in the normal BMI group [RBP4: (39.61 ± 1.57) mg/L vs (33.40 ± 1.28) mg/L, P < 0.01; FINS: (8.82 ± 3.79) mU/L vs (6.43 ± 4.38) mU/L, P < 0.05; HOMA-IR: 1.91 ± 0.85 vs 1.36 ± 0.72, P < 0.05]. (2) The expression levels of RBP4 mRNA and protein were significantly higher in omental adipose tissues than those in subcutaneous adipose tissue [mRNA: (5.88 ± 2.37) vs (2.07 ± 1.66), P < 0.01; protein: (0.76 ± 0.18 vs 0.15 ± 0.07, P < 0.05] and these depots difference in both groups (P < 0.05). Moreover, the overweight patients had the higher expression levels of RBP4 mRNA and protein in omental adipose tissue than normal BMI group (mRNA: 7.52 ± 0.58 vs 4.37 ± 0.45, P < 0.01; protein: 0.92 ± 0.23 vs 0.57 ± 0.13, P < 0.05). (3) The expressions of both RBP4 mRNA and protein in the omental adipose tissue were positively correlated with BMI, waist circumstance, serum concentrations of RBP4, FINS and HOMA-IR. The expression of was negatively correlated with HOMA-IR (r = -0.635, P < 0.05) in subcutaneous adipose tissue. No correlations were found between the expressions of RBP4 mRNA and protein in subcutaneous adipose tissue with BMI, waist circumstance, serum RBP4 and FINS concentrations. There is depot-specific expression of RBP4 in human subcutaneous and omental adipose tissues. A high expression of RBP4 in omental adipose tissue and an elevated level of serum RBP4 may contribute to the pathogenesis of obesity and IR.